Linux is a FREE alternative to the Windows XP/Vista that most of you use.
It does not suffer from viruses, trojans or any of the crap that you Windows users put up with.
any software bugs are fixed on average within 7 days where Microsoft take an average of 45 days.
99% of software that runs on it is FREE. you can also download the actual program code for pretty much everything and if you want(and have teh skill) change/improve/tweak it.
In order to help you get an idea of it it also runs happily on the same PC as windows, you get a menu when you stat up as to which OS you wish to use that time. recently they have even made it able to run "inside" windows so you can get an idea what it is like
have a look here
Ubuntu Home Page | Ubuntu
that is one version ( we geeks call them distros), there are other depending on what your individual requirements are but Ubuntu is a very good starters version. e.g There is one alled Peanut Linus that runs brilliantly on really old and low powered kit.
Linux is actually used on lots of things alraedy that you probably don't realise, BT Home hub runs it, some mobiles run it, anything from microwaves to personal video recorders can use it. teh whole point is that its totally configurabel to whatever you want to apply it to. I have an iPod that can run either the original Apple OS or Linux. the Linux version supports more types of music files, has extra games and can even record sound(which the original Apple one can't).